1. Field of the Invention
This invention relates to a method of reducing sulfur trioxide (SO.sub.3) concentration in the exit flue gas from the regeneration zone of catalytic cracking units. More particularly, this invention relates to a method of maintaining the SO.sub.3 /SO.sub.x ratio in the exit flue gas at a predetermined level.
2. Description of the Prior Art
Environmental limitations imposed by state and federal regulatory agencies are becoming increasingly important considerations in the operation of catalytic cracking units (e.g., fluid catalytic cracking-FCC units). In many areas of the country, and even in some foreign countries, economic penalties, (e.g., reduced throughput, more expensive raw materials) are being paid for the excessively high levels of pollutants produced in the catalytic cracking operations. Most of the gaseous pollutants, formed in a catalytic cracking operation, are produced in the regenerator zone or vessel. For example, typical FCC unit comprises a reactor zone or vessel with a catalyst and a regenerator vessel wherein spent catalyst is regenerated. Feed is introduced into the reactor vessel and is converted therein over the catalyst. Simultaneously, coke forms on the catalyst and deactivates the same. The deactivated (spent) catalyst is removed from the reactor zone and is conducted to the regenerator zone wherein coke is burned off the catalyst with an oxygen-containing gas (e.g., air), thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. Some of the catalyst is fractionated into fines and lost during the process because of constant abrasion and friction thereof against the various parts of the apparatus.
The efficiency of the regenerating operation is dependent on several operating parameters, the most important of which are regeneration temperature and oxygen availability. In recent years most operators have concentrated on rising regenerator temperature to increase the efficiency of the regenerator zone through a complete or almost complete combustion of carbon monoxide in the regenerator vessel. This is most commonly accomplished with the introduction of a carbon-monoxide combustion promoter usually comprising at least one of the following metals: platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), and rhenium (Re). Some new regenerator designs have incorporated better mixing methods for mixing coke catalysts with platinum and oxygen (e.g., fast fluidized bed regenerator of Gross et al, U.S. Pat. No. 4,118,338, the entire contents of which are incorporated herein by reference). However, while these new methods of operation of the regenerating vessel decrease the amount of carbon monoxide exiting with the flue gas, and improve the overall efficiency of the regeneration process, they may sometimes contribute to an increased production of other pollutants, e.g., sulfur oxides, particularly sulfur trioxide (SO.sub.3), and nitrogen oxides (see for example Luckenbach, U.S. Pat. No. 4,235,704).
Simultaneously with the improved methods of operation of a regeneration zone, which alone contribute to an increased production of sulfur oxides in the flue gases of the regenerator, sulfur feed levels in petroleum crudes available for cracking have been steadily increasing over the past few years. In the past, due to overall low levels of sulfur in FCC feeds, SO.sub.3 levels in flue gases were low, and generally only total SO.sub.x levels were monitored without an SO.sub.2 /SO.sub.3 breakdown or without regard to SO.sub.3 levels. With the combination of the high sulfur feed levels and the high temperatures in the regeneration zone, the SO.sub.3 concentration in the flue gas can be high enough to cause condensation in the flue gas which can result in a visible plume. Although all SO.sub.x emissions eventually turn to SO.sub.3 in the atmosphere and fall to earth as acid rain, there are environmental reasons for preferring the emissions to be sulfur dioxide (SO.sub.2), and the reaction of SO.sub.2 to SO.sub.3 to be carried out over an extended period of time. For example, high SO.sub.3 concentrations resulting in a visible plume can fall to earth in a small area and cause more environmental damage than highly dispersed acid rain. In addition, various state and federal regulatory agencies presently set a maximum limit on the amount of SO.sub.3, individually or as a function of the total SO.sub.x emissions being discharged from an industrial plant. (The term, total SO.sub.x emissions, as used herein means the sum total of the concentration of all sulfur oxides in a given gaseous stream.) Thus, restrictions are usually more stringent with respect to the sulfur trioxide emissions than they are for the sulfur dioxide emissions. For example, the state of New Jersey imposes a maximum of 2,000 parts per million (ppm) by volume for SO.sub.2 emissions and 85 ppm by volume for the SO.sub.3 emissions.